1. Field of the Invention
The present invention relates generally to a signal compensating apparatus which is used for an information reproducing apparatus that reproduces information recorded on a recording medium by using a light beam or an information recording apparatus that records information onto a recording medium, and specifically to a signal compensating apparatus which can compensate a reproduction signal damaged by a defect such as a crack on the recording medium and the like.
2. Description of the Related Art
Generally, the information reproducing apparatus reproduces the information recorded on the recording medium such as an optical disk, while carrying out a focus servo control and tracking servo control. The information reproducing apparatus carries out the focus servo control by using a light beam reflected by the information record surface of the optical disk in order to focus the light beam on the information record surface. Furthermore, the information reproducing apparatus carries out the tracking servo control by using the light beam reflected by the information record surface in such a way that the light spot of the light beam is located on an information track(s) formed on the optical disk.
In the focus servo control, the information reproducing apparatus generates a focus error signal by using the reflected light beam. This focus error signal includes information with respect to a deviation of the focal point from the information record surface. The information reproducing apparatus detects the deviation of the focal point on the basis of the focus error signal, and carries out the focus servo control so as to eliminate the deviation.
In the tracking servo control, the information reproducing apparatus generates a tracking error signal by using the reflected light beam. This tracking error signal includes information with respect to a difference between the light spot position and the information track position on the information record surface. The information reproducing apparatus detects this difference on the basis of the tracking error signal, and carries out the tracking servo control so as to eliminate this difference.
If there is a defect or obstruction, such as various defect of the optical disk (e.g. a damage of the information pit), cracks, dusts existing on the way of the optical path of the light beam or on the optical disk, etc., it becomes difficult to carry out the focus or tracking servo control correctly. Such a defect or obstruction (Hereinafter, it is referred to as a "defect".) has influence on the reflected light beam. As a result, a reproduction signal is partly damaged. More concretely, in the damaged part of the reproduction signal, the level of the reproduction signal decreases below the lower limit of the normal level range of the reproduction signal. This decrease of the level of the reproduction signal leads to a distortion of the wave form of the error signal (i.e. focus error signal or tracking error signal). As a result, accuracy of the servo control decreases.
In the conventional information reproducing apparatus, in order to solve this problem, the distortion of the error signal caused by the defect is detected, and the voltage level of a normal part of the error signal ahead of the distortion part is held by a sample-hold circuit or the like. That is, the distortion part is replaced by the constant voltage.
Furthermore, when reproducing the information recorded on the optical disk, the information reproducing apparatus operates on the basis of a standard reproduction clock signal. The standard reproduction clock signal is generated on the basis of information obtained from the reflected light beam. In this case, if there is a defect, the standard reproduction clock signal is damaged by this defect. More concretely, the standard reproduction clock signal is generated by the PLL (Phase Locked Loop) circuit. If there is a defect, the wave form of a control voltage generated by a VCO (voltage Control Oscillator) of a PLL is distorted due to this defect.
In order to eliminate the influence of the defect on the control voltage, the aforementioned method, namely, the method where the distortion part is replaced by the constant voltage is used.
However, the aforementioned method causes another problem. In the aforementioned information reproduction apparatus, when the distortion part has been eliminated by replacing the distortion part by the constant voltage, the constant voltage is replaced again by the original signal, i.e. the normal error signal. At this time, there is a possibility that a large difference exists between the constant voltage and the voltage of the error signal. This is because the actual error signal is varies while the constant voltage is being output.
If the large difference exists between the constant voltage and the voltage of the error signal, the level of the error signal, as a result, sharply varies. If such an error signal is fed into a focus actuator or a tracking actuator, which is used for the focus servo control or the tracking servo control, the actuator is suddenly driven. This causes the actuator to be unstable. As a result, accuracy of the servo control decreases.
This problem is serious from the viewpoint of the improvement of a record density in recent years (a reduction of an allowable value in a control deviation resulting from a contraction of an interval of the information track and a miniaturization of the information pit).
In the case of generating the standard reproduction clock signal, the similar problem occurs. If the large difference exists between the constant voltage and the actual control voltage generated by the VCO of the PLL, the actual control voltage, as a result, sharply varies, and therefore, serious errors occurs in a process of reproducing the information recorded on the optical disk. Such serious errors cannot be corrected by using the error correction technique. This results in a problem that it may be impossible to reproduce the information correctly.